The present invention relates to a method of locating assembly points in an assembly process, particularly but not exclusively, a method of marking out drilling locations in an industrial assembly process, such as aircraft assembly.
In conventional large scale assembly processes such as are employed in the aircraft industry, or dockyards, there is a frequent requirement to fix parts to large structures.
In the case of aircraft assembly, for example, a wing box for a wing of a large passenger liner may be up to 30 meters in length. Because of the great size of the structure, measuring a position on the structure to a high degree of accuracy is difficult to achieve. When a wing skin is to be fixed to such a wing box, and the two are clamped together prior to fixing, it is essential to determine accurately from the wing skin side of the structure where to drill attachment holes through the wing skin and into the supporting feet of a rib of the wing box.
This process is conventionally achieved in several separate operations. Firstly, guide holes of a smaller than final diameter may be drilled in the rib feet in the desired locations, prior to offering up the wing skin. Secondly, with the wing skin in place, pilot holes are drilled from inside the wing box outwards through the wing skin, in a process known as “back drilling”. Thirdly, using the pilot holes, the position of the predrilled guide holes in the rib feet are estimated. Finally, drilling of assembly holes from the outside of the wing skin through the wing skin and into the supporting rib feet may be commenced.
However, if the orientation of a guide hole, with respect to the local wing skin surface, is estimated insufficiently accurately, the assembly hole may not fully circumscribe the guide hole drilled in the rib foot. This results in a “pipped” hole. Consequently, it may be necessary to re-drill the hole using an oversized drill bit, in order to rectify the “pipped” hole. However, where the structure being assembled is a stressed structure, the effect of oversized drilling may give rise to a reduced service life of the structure.